This may be just sheer incompetence but some insiders are saying it's specifically because it went Net Zero compliant in 2012 and switched diesel generators for biomass ones.

Apparently some terrorism investigators are looking closely at the cause of the fire, but the crazy thing is, that Heathrow was completely reliant on one substation nearby which went up in a fireball yesterday, and never had even the slightest hope of keeping the airport running. The blackout left something like 290,000 people stranded, with 1,3oo flights canceled or rearranged. A bit like a war broke out or a volcano exploded, except they didn't.

Fury as Heathrow's lack of back-up power causes 'a contained version of 9/11': Small fire at power station leaves more than 200,000 travellers, and others forced to turn around in mid-air
By Martin Robinson, Daily Mail

One industry source has claimed that Net Zero is to blame because Heathrow is moving from diesel back-up generators to biomass.

Reform MP Richard Tice said: 'It appears that Heathrow had changed its backup systems in order to be, wait for it…Net Zero compliant'.

'They had got rid of their diesel generators and had moved towards a biomass generator that was designed not to completely replace the grid but work alongside it. Their net zero compliant backup system has completely failed in its core function at the first time of asking'.

"Basically their net zero-compliant back-up system had completely failed in its core function at the first time of asking. It beggars belief."

Speaking to The Telegraph, Mr Tice said: "Why is Heathrow being so silent about this? Are they embarrassed because they have something to hide?

But they did win a Sustainability Leaders Award in 2013 for burning down 13,000 tons of forest.
They were proud of their effort to change the global weather by (0.0 degrees)

According to the DailyMail the North Hyde substation fire involved a very old transformer catching alight and "involved 25000 litres of cooling oil igniting". Apparently it burnt the generator at North Hyde and also the backup. Previously it had been running at 106% of capacity, suggesting they were pushing old equipment past its maintenance schedule.